Make sure you cut just a little outside of your described circle and that you buy snips that are for the direction you want to cut. There are left and right hand versions.
If you take your time, you will get a very accurate circle . There will be a few rough edges and a few marks from the serration of the blades but it all cleans up with some light filing, a fine grinding wheel(maybe) and some emery paper or fine waterproof paper. ** the edge of the cymbal will have some very sharp spots after trimming, so don't be inquisitive and run your finger over it before you have cleaned it up a bit**